This invention relates to the field of instrumentation for measuring the physical properties of thin films, such as those used in the microelectronics industry. More particularly, this invention relates to the measurement of film topography and thickness using an x-ray reflectivity system.
X-ray reflectivity is a technology that can be used to determine physical properties of a layer on a substrate. For example, using x-ray reflectivity, the density, thickness, and surface roughness of a layer can be determined. Such information tends to be very useful in some industries, such as in the microelectronics industry, where the proper formation of various layers and the close control of the processes by which they are made are vital to successful fabrication of the devices in which the layers are formed.
X-ray reflectivity measurements are made by directing an x-ray beam toward the layer at an incident angle. As the x-rays reflect off of the layer, they are received by a detector of some type, such as a scintillation screen. The angles at which the x-rays reflect off of the layer, and the intensity of the reflected rays at those different angles, contains information about the physical properties of the layer, such as the thickness, roughness, and density, as mentioned above.
There are several parameters in regard to the construction and operation of an x-ray reflectometer that are important in order to get accurate results. One parameter is the x-ray throughput of the x-ray reflectometer. In other words, the x-ray reflectometer must be constructed so that a sufficient number of x-rays reflect off of the sample and are received by the detector, or the detector is unable to produce strong enough signals for a valid reading. Another important parameter is the angular resolution of the x-ray reflectometer. Angular resolution is the degree to which the x-rays that reflect off of the surface of the layer at different angles are separated across the detector. In other words, if two reflected beams having different reflectance angles are read as having the same reflectance angle by the detector, because for example they are so close together that the detector cannot resolve the angular difference between them, then an inaccurate result will be reported by the reflectometer.
Some applications are relatively tolerant as to a reduced x-ray throughput, and other applications are relatively tolerant as to a reduced angular resolution. Ideally, it would be beneficial to be able to adjust x-ray throughput and angular resolution as desired. Unfortunately, the layout of x-ray reflectometers tends to make it extremely difficult to adjust either of these two parameters.
There is a need, therefore, for an improved x-ray reflectometer design that, for example, enables angular resolution and x-ray throughput to be more easily adjusted.
The above and other needs are met by an apparatus adapted for sensing characteristics of a layer disposed substantially within a plane, without making physical contact with the layer. An x-ray source produces x-rays, where the x-ray source has an axis disposed substantially perpendicular to the plane of the layer. A curved x-ray is reflector has an axis disposed substantially perpendicular to the plane of the layer. The x-ray reflector receives the x-rays from the x-ray source and directs the x-rays received to a target spot on the layer at angles whereby the x-rays reflect off of the layer as reflected x-rays at a reflection angle. The reflected x-rays have properties that are indicative of the characteristics of the layer.
A first x-ray blocking barrier is disposed substantially perpendicular to the plane of the layer, above the target spot. The first x-ray blocking barrier blocks at least a portion of the x-rays directed toward and reflected off of the layer. The first x-ray blocking barrier and the layer define a gap, where the size of the gap determines at least in part a throughput and an angular resolution of the x-rays reflected off the layer. A receptor receives the reflected x-rays and produces signals based on the properties of the reflected x-rays. An analyzer receives the signals from the receptor and determines the characteristics of the layer based at least in part on the properties of the reflected x-rays.
In this manner, the angular resolution can be increased by reducing the size of the gap, or alternately, the x-ray throughput can be increased by increasing the size of the gap. Thus, control over the characteristics of the system is available through adjusting the gap. This is made possible in part because the x-ray source and the x-ray reflector have an orientation relative to the layer that is different from that of prior art x-ray systems.
In various preferred embodiments, a second x-ray blocking barrier is disposed substantially perpendicular to the plane of the layer, between the x-ray reflector and the layer. The second x-ray blocking barrier blocks at least a portion of any x-rays that are not directed to the target spot on the layer. Preferably, the x-ray reflector is comprised of at least one of silicon, germanium, and lithium fluoride, and most preferably is formed of a single crystal structure, where a crystal plane of the single crystal structure has a curvature defined along the axis of the x-ray reflector and the crystal plane of the single crystal structure also has a curvature defined around the axis of the x-ray reflector.
Preferably, a surface of the single crystal structure of the x-ray reflector has a cylindrical curvature defined around the axis of the x-ray reflector. In one embodiment, the x-ray reflector is formed of a single crystal structure, where a surface of the single crystal structure has an elliptical curvature defined both around and along the axis of the x-ray reflector. The receptor is preferably a charge coupled device array. The x-ray source preferably produces a divergent cone of x-rays directed toward the x-ray reflector, which in turn produces a convergent cone of x-rays substantially focused on the target spot of the layer. Most preferably the x-ray source is a linear focus x-ray tube.
In an alternate embodiment, the x-ray reflector is a Johnson geometry crystal. The size of the gap is preferably adjustable from about ten microns to about one hundred microns. A movable stage preferably selectively raises and lowers the layer relative to the first x-ray blocking barrier, and thereby adjusts the size of the gap. A sensor preferably determines the size of the gap. In a most preferred embodiment, the linear focus x-ray tube produces copper Ka x-rays from a source having dimensions of about twelve millimeters by about forty microns, and the spot on the layer is about forty microns in width.
In one embodiment the apparatus includes a second x-ray source having an axis and disposed above the linear focus x-ray tube. The axis of the second x-ray source is substantially parallel to the axis of the linear focus x-ray tube. A second x-ray reflector with an axis is disposed above the x-ray reflector. The axis of the second x-ray reflector is substantially parallel to the axis of the x-ray reflector. The second x-ray source and the second x-ray reflector increase the range of reflection angles measured by the charge coupled device array receptor, and thus increase the angular resolution of the apparatus.
In an alternate embodiment, the apparatus includes a second x-ray source with an axis, disposed beside the linear focus x-ray tube. The axis of the second x-ray source is substantially parallel to the axis of the linear focus x-ray tube. A second x-ray reflector with an axis is disposed beside the x-ray reflector. The axis of the second x-ray reflector is substantially parallel to the axis of the x-ray reflector. The second x-ray source and the second x-ray reflector increase the x-ray throughput.